I’m a consultant and seminar leader who specializes in the graphical display of data. I train employees of corporations and organizations on effective data visualization. I also review documents and presentations for clients, suggesting improvements or alternative presentations as appropriate. I’m the author of Creating More Effective Graphs, published by Chart House in 2013 (reprinted from Wiley 2005). In addition to my one and two day seminars on creating more effective graphs, I offer short programs such as “Recognizing Misleading and Deceptive Graphs” and “How to Avoid Common Graphical Mistakes.” I received a Ph.D. in mathematical statistics from Columbia University, M.A. from Cornell University, and A.B. from Bryn Mawr College. I had a long career at Bell Laboratories before forming NBR, my consulting practice.

Visualizing Stop-and-Frisk and Murder Rates in New York City

Debate over the New York Police Department’s controversial stop-and-frisk policy has intensified recently with an editorial attack by The New York Times and an NYPD rebuttal. One of many major points of contention is whether this tactic can be credited with reducing the NYC murder rate. The NYPD and its supporters have repeatedly stressed that stop-and-frisk is part of a policy that saves lives. The Daily Newsreports that according to the NYPD’s top spokesman, Deputy Commissioner Paul Browne, “Over the past 10 years, there were 5,430 murders in New York City, compared with 11,058 in the decade before Mayor Michael Bloomberg took office.” Police Commissioner Ray Kelly directly links a significant drop in the city’s murder rate with the stop-and-frisk policy.

Is it so? Despite all the talk of declining crime and increased numbers of stop-and-frisks, are the two connected? The short answer is no! All of the graphs in today’s post make it clear that the astronomical increase in stop-and-frisks came well after the significant decrease in number of murders, and thus cannot be the cause of the drop.

I present these different versions as a means to discuss choices and considerations when plotting two trends simultaneously. To compare, the most straightforward method is simply to plot the number of murders and the number of stop-and-frisks, as I did in Figure 1. This clearly shows that the number of murders decreased sharply between 1990 and 1998 while the number of stop-and-frisks had a sharp increase beginning in 2002. (Stop-and-frisk data are not available for years prior to 2002.)

Figure 1. The numbers of murders and the number of stop-and-frisks in New York City.

I chose two panels since the magnitudes for the two measures are not the same, and plotting them on the same set of axes would hide any variation in the numbers of murders over time. Although I often recommend including units in axis labels rather than cluttering a figure with repeated percent signs, dollar signs or other units, I deviated from this recommendation here in the y-axis labels–note the multiple 0′s and K’s–to emphasize the fact that the scale for stop-and-frisks is in thousands while the scale for murders is not.

Figure 2. The number of murders and the number of stop-and-frisks are shown on the same panel with dual axes. This figure is not my first choice as dual axes may mislead viewers.

To demonstrate the problem with dual-axis figures, I plotted both trends in the same panel, shown in Figure 2. Although it clearly shows the time gap between the decrease in murders and the increase in stop-and-frisks, the left and right scales are independent so the designer can give many different impressions of the data by manipulating the scales. Also, readers place meaning on the point where the curves cross when no meaning exists since the relationship of the two scales is arbitrary. For these reasons, this figure is not my first choice.

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The eight-grade math of massively frisking New York City’s minorities: After — crime was reduced by 4X, stop-and-frisk went up by 6X = 24X as many frisks per reported crime.

This means — either — that those police officers and administrators responsible for reducing crime to 25% of what it was were totally remiss and neglectful in their patrol duties (wouldn’t that be a contradiction in terms?) — or — that the Fourth Amendment has been totally suspended for New York City minorities (the mayor’s intended victims?).

Could 24X increase in frisking be a move by Mad King Bloomberg* to reduce the numbers of the less affluent and minorities — to make room for more of their opposite numbers? ???

* The allusion is to Mayor Bloomberg wasting a half billion dollars (in today’s money) to build a massive new courthouse in the Bronx — to replace the beautiful courthouse at the corner of 161st street and Grand Concourse, a courthouse so beautiful outside and functional inside they cannot tear it down (it’s the black and white striped building you used to see in the background looking to the outfield of the old Yankee Stadium) and the brand new,when I was going there in the late 1970s, courthouse down the hill; two of the nicest courthouses in the country — in, until the recent incursion of yuppies, the poorest congressional district in the country — a half billion dollars!

Stop and frisk itself may not have much to do with the decreasing murder rate but this all started with Giuliani’s”Quality of life” campaign of which ‘Stop and frisk’ is part of. NYC was an absolute cesspool in the 70′s and 80′s and only after about two or three years of Giuliani did things start to get better.

But the stop and frisk is a direct result of police calls and suspect searches. If anyone of you were to pick up a police scanner you’ll find out the ugly truth that the majority of the criminals doing the crimes are black and Latino. That’s what drives the NYPD to use a wide net when profiling everyone.

I’m Latino myself and the victim of various crimes in this city from time to time and those times they have been by black people. I’m all for stop and frisk because honestly some of these people shouldn’t be allowed live let alone on the streets. Lock them up and throw away the key. We’d all be better off.

If Ms. Robbins’s premises were valid, her conclusions might also be valid. As it is, though, her premises are completely invalid. Ms. Robbins cannot accurately state with bland assurance that “the number of stop-and-frisks had a sharp increase beginning in 2002″. First of all, as she herself admits, she has no data at all from before 2002. More to the point, though, is that she is making the completely invalid assumption that the number of completed stop-and-frisk forms (a document referred to by NYPD officers as a “UF 250″) is the same as the number of stop-and-frisks. It is in making this assumption that Ms. Robbins’s analysis crashes and burns.

Ms. Robbins appears to be unaware that the completeion of a UF 250 was traditionally OPTIONAL, rather than mandatory, when a stop-and-frisk occurred. The vast majority of stop-and-frisks conducted during the initial years of New York’s great decline in crime resulted in the completion of *no* UF 250 whatsoever, and therefore no one (or at least, no honest person who cares for the accuracy of his or her figures) can say with any accuracy how many stop-and-frisks were conducted at that time, and how many it would take to indicate an increase or a decrease. The sudden increase in numbers that Ms. Robbins notes is not an increase in stop and frisks, but an increase in UF 250s. The increase indicates nothing whatsoever about the number of stops and frisks being conducted, but instead indicates that NYPD procedures changed in a way that made the completion of a UF 250 mandatory rather than optional. Subsequently, and for the first time, the submission of a UF 250 was taken as an indication of a officer’s activity in the field in the same way that arrests and the issuance of summonses were. As police officers now had a real incentive to submit completed UF 250s, the number submitted naturally increased. It is thus as invalid to say that the lack of paper UF 250s before a certain date should be taken as showing a corresopnding lack of stop and frisks as it would be to think that a lack of paper death certificates for victims of the Black Death shows that no one died of plague in Europe during the 1340s.